An information processing apparatus comprises various registers for informing a system of a hardware structure or data format, and having specific areas on a memory for administrating processing status by the system.
If the contents of the routine followed by the registers are unduly broken off, the system does not work properly. Even if control is shifted to an external processing such as by an interruption, the contents of the registers can be utilized even at a routine for a jumped destination.
In an ordinary system, however, a former status is broken off, because the processing for the jumped destination uses the registers at the routine. Therefore, when the processing shifts to an external routine, the contents of the registers are preserved, and the control must be returned to the original routine, after the preserved registers are restored to the original state at the time of the return to the original routine.
FIG. 1 shows the shift of controls at an interruption process, and the state of registers at that time. When the interruption occurs, the control of a CPU shifts to an interruption routine. When the interruption finishes, the control usually returns to an instruction following an instruction which caused the interruption.
At this time, the registers are restored to their state at which the interruption started, so that the processing of the original routine continues with the ordinary operation routine subsequent to operation which occurred just prior to the start of the interruption. The preservation of the registers in the state they were in at the interruption routine is carried out for only the registers to be used at the interruption routine. There is no necessity to carry it out for all registers.
At any rate, a conventional information processing apparatus is provided with a memory having a work area for preserving the contents of the registers at interruption routine, wherein the preserved contents are broken off in the work area at the end of the interruption routine. For this reason, the store of the contents and the restore thereof are repeated in the work area each time there is an interruption routine.
In the case of a computer game machine, subsequent image process must be carried out during V and H blanks (vertical and horizontal retrace periods) having no image on a video screen. Even more, the process is an interruption process such as V blank interruption, H blank interruption, etc.
FIG. 2 shows an image display on a TV screen realized by scanning lines.
The scanning line runs left to right, and, when it reaches to the right end, it returns to the left end during a horizontal retrace period having no displayed image on the screen. On the other hand, the scanning line moves from the top to the bottom of the displayed image. When it reaches the bottom, it returns to the top of the image during a vertical retrace period having no displayed image on the screen.
Even a relatively long V blank occurring once in a 1/60 sec. period has a period of 3/242 sec. The H blank is extremely short as approximately 10M sec. Therefore, a time for the store and restore of registers is not negligible, because the processing of images to be next displayed must be done in these short periods. Therefore, there is a disadvantage in the conventional information processing apparatus in that a part of the process to be carried out in the retrace periods is omitted.